The Mysterious Case of the Musca Domestica Contaminant

Just when I thought I’d seen everything that could contaminate the background, a user sent me this picture! The Musca Domestica (common housefly) was attracted by certain aromatics used in testing and seems to have entered the measurement cell through an empty Hydro EV over a weekend. Unfortunately, it became wedged within the measurement cell and did not survive the experience.

The most important message from the Mastersizer training courses I give is to make sure you have a good background before you start. Laser diffraction is a background subtracted technique: the light scattered by the dispersant, the cell windows and the optical path is measured and subtracted from the signal read on the detectors. The background is measured each time a SOP or manual measurement is run.

The background should be stable and

Detector 1 should be less than 100

Detector 20 should be less than 20

The profile should be a decreasing curve as shown below.

The measurement can only be as good as the sample preparation and the background. If the background is poor, results are likely to be poor also. The most common causes of poor backgrounds are not allowing the background to stabilize, contamination, and material on the cell windows. I now need to add houseflies to my contamination examples.

See a complete measurement in the Mastersizer 3000 with Hydro EV accessory in this video